Phase change electrophoretic imaging for rewritable applications

ABSTRACT

The present invention relates to a microcapsule that can be used to form a rewritable medium for visual displays that are stable in the presence of electric fields having a strength that is typical in the work environment. The microcapsule of the invention comprises charged particles of one or more colors that are suspended in a phase change material that has a melting temperature in the range of between about 30° C. and about 200° C. The microcapsules can be used to form an electrophoretic coating that includes microcapsules of the invention distributed throughout a polymer matrix. The electrophoretic coating can be used to coat a substrate to form a rewritable medium.

This is a division of application Ser. No. 10/792,335 filed Mar. 2, 2004now U.S. Pat. No. 6,970,285.

BACKGROUND

Flexible displays made with a technology known as electronic ink, or Eink, exhibit good brightness and contrast, a wide viewing angle andlittle or no power consumption are currently being exploited forapplications in which a portable reusable display medium is necessary,such as in reusable paper. One way to make a reusable medium that isportable is to remove the driving electronic from the electronic displayand use external addressing electrodes to write and erase images.

Generally, an encapsulated electrophoretic display includes one or morespecies of particles that either absorb or scatter light. One example isa system in which the capsules contain one or more species ofelectrophoretically mobile particles dispersed in a dyed suspendingmedium. Another example is a system in which the capsules contain twoseparate species of particles suspended in a clear suspending fluid, inwhich one of the species of particles absorbs light (black), while theother species of particles scatters light (white). Other extensions arepossible, including more than two species of particles, with or withouta dye, etc. The particles are commonly solid pigments, dyed particles,or pigment/polymer composites.

The gyricon, also called the twisting-ball display, rotary ball display,particle display, dipolar particle light valve, etc., offers atechnology for making a form of electric paper. A gyricon is anaddressable display made up of a multiplicity of optically anisotropicballs, each of which can be selectively rotated to present a desiredface to an observer. Thus, in one version at least, the gyricon is asolid microsphere, hemispherically-colored black and white and havinghemispherically-opposing zeta potentials. Each gyricon rotates within adielectric oil-filled microcavity formed in the media upon exposure toan externally-applied electric field.

Unfortunately, currently available portable reusable displays sufferfrom problems with long term stability and can be destabilized by anelectric field or even by static charge that builds up during normalhandling. Therefore, in order to make portable reusable displays morereliable and convenient to use, it would be desirable to have a portablereusable display medium that is stable in the presence of an electricfield and, in particular, is not destabilized by electrostaticdischarge.

SUMMARY

The present invention relates to a microcapsule that can be used to forma rewritable medium for visual displays that are stable in the presenceof electric fields having a strength that is typical in the workenvironment, and, in particular, the rewritable medium of the inventionis stable to electrostatic discharge. The microcapsule of the inventioncomprises charged particles of one or more colors that are suspended ina phase change material. The phase change material has a meltingtemperature in the range of between about 30° C. and about 200° C. Themicrocapsules can be used to form a rewritable medium comprising anelectrophoretic coating on a substrate. The electrophoretic coatingincludes microcapsules of the invention that are distributed, preferablyuniformly, throughout the polymer matrix. In one embodiment, themicrocapsules have at least two particles and particles of one or morecolors have a positive charge and particles having one or more differentcolors have a negative charge.

An image can be formed on a rewritable medium of the invention using anapparatus that includes a heating element, at least one electrode, ameans for positioning the heating element to heat a section of theelectrophoretic coating, and a means for positioning the electrode abovethe surface of the section of the electrophoretic coating that is beingheated or has been heated by the heating element.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic representation of an embodiment of the inventionwith a microcapsule of the inveintion after the phase change medium hasbeen liquefied by heating but before an electric field has been applied.

FIG. 1B is a schematic representation of an embodiment of the inventionwith the microcapsule of FIG. 1A after an electric field has beenapplied.

FIG. 2A is a schematic representation of one embodiment of the inventionwith an apparatus for forming an image on a CD or DVD having arewritable electrophoretic coating.

FIG. 2B is a cross-sectional view along line (I) of the CD or DVD in theapparatus of FIG. 2A according to an embodiment of the invention.

FIG. 3 is a schematic representation of one embodiment of the inventionwith an apparatus for forming an image on a rewritable medium having anelectrophoretic coating.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

The invention will now be described with particular reference to certainpreferred embodiments of the microcapsules and rewritable medium of theinvention.

The present invention relates to a microcapsule that can be used to forma rewritable medium for visual displays that are stable in the presenceof electric fields having a strength that is typical in the workenvironment. The microcapsule of the invention comprises chargedparticles of one or more colors that are suspended in a phase changematerial that has a melting temperature in the range of between about30° C. and about 200° C. In one embodiment, the phase change materialhas a melting point in the range of between about 60° and about 100° C.The microcapsules can be used to form an electrophoretic coating thatincludes microcapsules of the invention distributed, preferablyuniformly, throughout a polymer matrix. The electrophoretic coating canbe used to coat a substrate to form a rewritable medium. The substratemay be any solid material, such as a plastic or paper, provided that thesubstrate does not melt or decompose at a temperature needed to melt thephase change material.

The choice of particles for use in the microcapsules of the rewritablemedium of the invention is not restrictive. However, the particlesshould not be soluble in the phase change material. The term “chargedparticles,” as used herein, refers to particles that are charged orcapable of acquiring a charge (i.e., has or is capable of acquiringelectrophoretic mobility). In one embodiment, the particles are coatedwith a surfactant thereby adding a surface charge to the particles.Particles may be neat pigments, dyed (laked) pigments or pigment/polymercomposites, or any other component that is charged or capable ofacquiring a charge. Typical considerations for the electrophoreticparticle are its optical properties, electrical properties, and surfacechemistry. The particles may be organic or inorganic compounds, and theymay either absorb light or scatter light. Particles useful in therewritable medium of the invention may further include scatteringpigments, absorbing pigments and luminescent particles. Particles may beretroreflective, such as corner cubes, or they may beelectroluminescent, such as zinc sulfide particles, which emit lightwhen excited by an AC field, or they may be photoluminescent. In oneembodiment, particles are surface treated so as to improve charging orinteraction with a charging agent, or to improve dispersibility.

In one embodiment, particles useful in the rewritable medium of theinvention are titania. Titania particles may be coated with a metaloxide, such as aluminum oxide or silicon oxide, for example. Titaniaparticles may have one, two, or more layers of metal-oxide coating. Forexample, a titania particle for use in rewritable medium of theinvention may have a coating of aluminum oxide and a coating of siliconoxide. The coatings may be added to the particle in any order. Otheruseful particles include barium sulfate, kaolin, zinc oxide and carbonblack.

The electrophoretic particle is usually a pigment, a polymer, a lakedpigment, or some combination of the above. A neat pigment can be anypigment, and, usually for a light colored particle, pigments such as,for example, rutile (titania), anatase (titania), barium sulfate,kaolin, or zinc oxide are useful. Some typical particles have highrefractive indices, high scattering coefficients, and low absorptioncoefficients. Other particles are absorptive, such as carbon black orcolored pigments used in paints and inks. The pigment should also beinsoluble in the liquefied phase change material. Yellow pigments suchas diarylide yellow, hansa yellow, and benzidine yellow are also usefulfor the particles of the invention. Any other reflective material can beemployed for a light colored particle, including metallic particles.

Useful neat pigments include, but are not limited to, PbCrO₄, Cyan blueGT 55-3295 (American Cyanamid Company, Wayne, N.J.), Cibacron Black BG(Ciba Company, Inc., Newport, Del.), Cibacron Turquoise Blue G (Ciba),Cibalon Black BGL (Ciba), Orasol Black BRG (Ciba), Orasol Black RBL(Ciba), Acetamine Black, CBS (E. I. du Pont de Nemours and Company,Inc., Wilmington, Del.), Crocein Scarlet N Ex (du Pont), Fiber Black VF(duPont), Solvent Black 17, Nirosine Base No. 424 (duPont), SolventBlack 16, Rotalin Black RM (duPont), Sevron Brilliant Red 3 B (duPont);Basic Black DSC (Dye Specialties, Inc.), Hectolene Black (DyeSpecialties, Inc.), Solvent Blue 9, Solvent Green 2, Azosol FastBrilliant Red B (GAF), Solvent Orange 20, Azosol Fast Yellow GRA Conc.(GAF), Basic Black KMPA (GAF), Benzofix Black CW-CF (GAF), DisperseBlack 9, Disperse Blue 9, Basic Black3, Diamine Black CAP Ex Conc (GAF),Diamond Black EAN Hi Con. CF (GAF), Diamond Black PBBA Ex (GAF); DirectDeep Black EA Ex CF (GAF), Hansa Yellow G (GAF); Indanthrene Black BBKPowd. (GAF), Indocarbon CLGS Conc. CF (GAF), Katigen Deep Black NND HiConc. CF (GAF), Rapidogen Black 3 G (GAF) (Azoic Blk. 4); SulphoneCyanine Black BA-CF (GAF), Zambezi Black VD Ex Conc. (GAF); Rubanox RedCP-1495 (The Sherwin-Williams Company, Cleveland, Ohio); Raven 11(Columbian Carbon Company, Atlanta, Ga.),, Statex B-12 (Columbian CarbonCo.), and chrome green.

Particles may also include laked, or dyed, pigments. Laked pigments areparticles that have a dye precipitated on them or which are stained.Lakes are metal salts of readily soluble anionic dyes. These are dyes ofazo, triphenylmethane or anthraquinone structure containing one or moresulphonic or carboxylic acid groupings. They are usually precipitated bya calcium, barium or aluminum salt onto a substrate. Typical examplesare peacock blue lake (Cl Pigment Blue 24) and Persian orange (lake ofCl Acid Orange 7).

A dark particle may be constructed from any light absorbing material,such as carbon black, or inorganic black materials. The dark materialmay also be selectively absorbing. For example, a dark green pigment maybe used. Black particles may also be formed by staining lattices withmetal oxides, such latex copolymers consisting of any of butadiene,styrene, isoprene, methacrylic acid, methyl methacrylate, acrylonitrile,vinyl chloride, acrylic acid, sodium styrene sulfonate, vinyl acetate,chlorostyrene, dimethylaminopropylmethacryl-amide, isocyanoethylmethacrylate and N-(isobutoxymethacrylamide), and optionally includingconjugated diene compounds such as diacrylate, triacrylate,dimethylacrylate and trimethacrylate. Black particles may also be formedby a dispersion polymerization technique.

In the systems containing pigments and polymers, the pigments andpolymers may form multiple domains within the electrophoretic particle,or be aggregates of smaller pigment/polymer combined particles.Alternatively, a central pigment core may be surrounded by a polymershell. The pigment, polymer, or both can contain a dye. The opticalpurpose of the particle may be to scatter light, absorb light, or both.The density of the electrophoretic particle may be substantially matchedto that of the phase change material in the liquid state. As definedherein, a phase change material has a density that is “substantiallymatched” to the density of the particle if the difference in theirrespective densities is between about zero and about two g/ml. Thisdifference is preferably between about zero and about 0.5 g/ml.

Useful polymers for the particles include, but are not limited to:polystyrene, polyethylene, polypropylene, phenolic resins, Du Pont Elvaxresins, ethylene-vinyl acetate copolymers, polyesters, polyacrylates,polymethacrylates, ethylene acrylic acid, methacrylic acid copolymers,Nucrel Resins—Dupont, Primacor Resins—Dow Chemical, acrylic copolymersand terpolymers (Elvacite Resins, DuPont) and PMMA. Useful materials forhomopolymer/pigment phase separation in high shear melt include, but arenot limited to, polyethylene, polypropylene, polymethylmethacrylate,polyisobutylmethacrylate, polystyrene, polybutadiene, polyisoprene,polyisobutylene, polylauryl methacrylate, polystearyl methacrylate,polyisobornyl methacrylate, poly-t-butyl methacrylate, polyethylmethacrylate, polymethyl acrylate, polyethyl acrylate,polyacrylonitrile, and copolymers of two or more of these materials.Some useful pigment/polymer complexes that are commercially availableinclude, but are not limited to, Process Magenta PM 1776 (Magruder ColorCompany, Inc., Elizabeth, N.J.), Methyl Violet PMA VM6223 (MagruderColor Company, Inc., Elizabeth, N.J.), and Naphthol FGR RF6257 (MagruderColor Company, Inc., Elizabeth, N.J.).

The pigment-polymer composite may be formed by a physical process,(e.g., attrition or ball milling), a chemical process (e.g.,microencapsulation or dispersion polymerization), or any other processknown in the art of particle production. From the following non-limitingexamples, it may be seen that the processes and materials for both thefabrication of particles and the charging thereof are generally derivedfrom the art of liquid toner, or liquid immersion development. Thus, anyof the known processes from liquid development are particularly, but notexclusively, relevant.

Typical manufacturing techniques for particles are drawn from the liquidtoner and other arts and include ball milling, attrition, jet milling,etc. These methods are described in U.S. Pat. No. 6,067,185, the entireteachings of which are incorporated herein by reference.

Chemical processes such as dispersion polymerization, mini- ormicro-emulsion polymerization, suspension polymerization precipitation,phase separation, solvent evaporation, in situ polymerization, seededemulsion polymerization, or any process which falls under the generalcategory of microencapsulation may be used. A typical process of thistype is a phase separation process wherein a dissolved polymericmaterial is precipitated out of solution onto a dispersed pigmentsurface through solvent dilution, evaporation, or a thermal change.Other processes include chemical means for staining polymeric lattices,for example with metal oxides or dyes.

Microcapsules containing the phase change material and charged particlescan be made by any desired or suitable process. For example, methods forpreparing microcapsules are disclosed in, for example, U.S. Pat. Nos.4,087,376, 4,001,140, 4,273,672, 5,961,804, 2,800,457, 5,604,027. Othermethods of preparing microcapsule having charged particles include U.S.Pat. Nos. 2,800,457 and 2,800,458 which disclose a phase separationmethod of forming microcapsules from an aqueous solution; JapanesePatent Publication Nos. 38-19574, 42-446, and 42-771 which disclose aninterfacial polymerization method of forming microcapsules; JapanesePatent Publication No. 36-9168 and Japanese Patent Application Laid-OpenNo. 51-9079 which disclose an insitu method of forming microcapsulesbased on monomer polymerization; and British Patent Nos. 952807 and965074 which disclose a dissolution dispersion cooling method of formingmicrocapsules. The entire teachings of the above patents and publishedpatent applications are incorporated herein by reference. However, othermethods known to those skilled in the art of forming microcapsuleshaving charged particles may be used.

The phase change material may be directly dispersed or emulsified intothe polymer matrix (or a precursor to the polymer matrix) to form anelectrophoretic coating. In such a coating, the individualelectrophoretic phase change droplets dispersed in the polymer matrixmay be referred to as capsules or microcapsules even though no capsulemembrane is present.

Alternatively, the microcapsules of the invention may be encapsulated inmaterial that forms an outer wall around the phase change material,provided that the material can transmit sufficient light. The materialfor forming the outer wall of the microcapsule can be any materialprovided it is usable to produce the outer wall by means of a method forproducing microcapsules known in the art. Preferred materials areoptically transparent polymeric materials. In addition, the materialthat forms the outer wall of the microcapsule preferably has sufficientthermal stability so that it does not melt or degrade under temperatureswherein the phase change material is a liquid. Examples of suitablematerials for the outer wall of microcapsules of the invention includepolyvinyl alcohol, polyethylene, polyamide, polyester, polyurethane,polyurea, polyurethane, polystyrene, nitrocellulose, ethylcellulose,methylcellulose, melamine/formaldehyde resin, urea/formaldehyde resin,and copolymers thereof.

It is preferable that the volume of particles in the microcapsulecarrying a positive or negative charge is in the range of between about0.1% and about 20% of the total volume of the microcapsule. In addition,it is preferable that the total sum volume of all of the chargedparticles is in the range of between about 0.1% and about 40% of thetotal volume of the microcapsule. In some embodiments, the total sumvolume of all of the charged particles is in the range of between about0.1% and about 20%, about 5% and about 15%, and about 9% and about 11%of the total volume of the microcapsule. If the volume of the chargedparticles is lower than 0.1% of the total volume of the microcapsule,the contrast of the image is lower because the eye of the observer maybe able to distinguish oppositely charged particles that have adifferent color. However, movement of the particles is excessivelyimpeded if the total volume of the particles exceeds 40% or the volumeof the set of particles that are positively charged or the set ofparticles that are negatively charged exceeds 20% resulting in reducedperformance of the rewritable medium in response to application of anelectric field.

The diameter of the charged particle is preferably is in the range ofbetween about 10 nm to about 5 μm, as long as the particles are smallerthan the microcapsules. The diameter of the microcapsule is preferablyis in the range of between about 5 μm to about 400 μm.

The polymer matrix is typically a transparent or translucent material.Selection of the polymer matrix is dependent on what phase changematerial is used in the microcapsules since the polymer matrix must havea melting temperature that is higher than the phase change material. Inaddition, the polymer matrix should not substantially degrade at themelting temperature of the phase change material. Examples of suitablematerials for the polymer matrix include elastomers, such as SYLGARD®184, available from Dow Corning, Midland, Mich., Stauffer and WackerV-53 elastomer; acrylics; polyvinylalcohols; polyvinylacetates;polyurethanes; polysaccharides, including cellulose and cellulosederivatives; gelatin arabic; gum arabic; polyamides; urea-formaldehyderesins; melamine-formaldehyde resins; N-methyl pyrrolidone; N-vinylpyrrolidone; poly-2-hydroxyethylacrylate; latex compositions, typifiedby the Neorez® and Neocryl® resins (Zeneca Resins), Acrysol® (Rohm andHaas), Bayhydrol® (Bayer), and the Cytec Industries HP line, includinglattices of polyurethanes, occasionally compounded with one or more ofthe acrylics, polyesters, polycarbonates or silicones; epoxies;polyesters; and the like, as well as mixtures thereof. After themicrocapsules have been dispersed within the precursor of the polymermatrix, the precursor is cured by any desired or effective method, suchas application of heat, radiation (such as UV-radiation), chemicalcuring, or the like. One example of a specific process for providingelectrophoretic microcapsules dispersed within a polymer matrix isdisclosed in U.S. Pat. No. 6,067,185, the entire teachings of which areincorporated herein by reference.

The phase change material can be any material that has a melting pointin the range of between about 30° C. and 200° C. Preferably, the phasechange material has a melting point in the range of between about 60° C.and about 100° C. Examples of useful phase change materials includeparaffin wax (melting point of about 65° C. to about 80° C.),1-docosanol (melting point of about 65° C. to about 72° C.),1-hexacosanol (melting point of about 80° C.), n-tetratetracontane(melting point of about 86° C.), 1-triacontanol (melting point of about88° C. to about 90° C.), and n-pencontane (melting point of about 94°C.). Preferred properties for phase change materials are shown in Table1.

TABLE I Preferred properties for phase change materials. PropertyPreferred Value Density Similar density to dispersed particlesRefractive Index <1.2 Refractive Index Difference <0.3 (more preferablyabout 0.05 to about 0.2) Dielectric Constant About 2 Volume ResistivityAbout 10¹⁵ ohm-cm Viscosity as a Liquid 5 cst Toxicity Low WaterSolubility <10 ppm Specific Gravity >1.5 Melting Point about 60° C. toabout 100° C.

In one embodiment, a material that absorbs infrared radiation may bedispersed throughout the phase change material in order to facilitatequickly heating the phase change material to above its melting point. Inone embodiment, (2-[2-[2-chloro-3-[(1,3-dihydro-3,3-dimethyl-1-propyl-2H-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-propylindoliumiodide), a dye that absorbs infrared radiation, is dispersed throughoutthe phase change material.

During or after application of sufficient heat to a section of theelectrophoretic coat to cause the phase change material in themicrocapsules to melt, an electric field is applied to theelectrophoretic coat. The electric field gradient is preferablyapproximately perpendicular to the surface of the electrophoreticcoating. However, other orientations of the electric field are possible.

In one embodiment, the microcapsules of the electrophoretic coating havecharged particles of at least two colors. In this embodiment, particleshaving one or more colors have a positive charge and particles havingone or more different colors have a negative charge. For example,microcapsules may contain negatively charged black particles andpositively charged white particles. In this example, the effect ofapplication of an electric field to a microcapsule 10 in anelectrophoretic coating that has been heated to a temperature that ishigh enough to liquefy the phase change material will be explained withreference to FIGS. 1A and 1B. When the electric field is not exerted,the negatively charged black particles 12 a and the positively chargedwhite particles 12 b are dispersed randomly or in a disordered mannerthroughout the phase change material 14, as shown in FIG. 1A. When theelectric field is applied to the electrophoretic coating in a direction,for example, perpendicular to the surface of the electrophoretic coatingas shown in FIG. 1B, the negatively charged black particles 12 a and thepositively charged white particles 12 b move in opposite directionsunder the influence of the electric field causing the black particles toaccumulate on one side of the microparticle closest to the positivelycharged electrode and the white particles to accumulate on the otherside of the microparticle closest to the negatively charged electrode.

The section of the electrophoretic coating is then allowed to cool whilecontinuing the application of the electric field or shortly afterdiscontinuation of the electric field so that the phase change materialsolidifies, trapping the black particles at one side of the microcapsuleand the white particles at the other side of the microcapsule. When themicrocapsule is viewed from above the surface of the electrophoreticcoating, the black particles, which are gathered on the lower side,cannot be seen because they are hidden by the white particles.Therefore, the observer will see this section of the rewritable mediumas white. If an electric field is applied in the opposite direction(i.e., the positive and the negative electrodes are reversed), thesection of the rewritable medium would be seen as black. Therefore, apredetermined image can be formed on the rewritable medium by applyingheat and changing the direction of the electric field in differentsections of the rewritable medium.

In a preferred embodiment of the invention, a multi-color,electrophoretic display is contemplated. In this embodiment, anelectrophoretic coating comprises a polymer matrix that hasmicrocapsules that have at least two, and preferably at least three,species of particles in the phase change material. These particles areof different colors and possess substantially non-overlappingelectrophoretic mobilities. As used herein, the phrase “substantiallynon-overlapping electrophoretic mobilities” means that less than 25%,and preferably less than 5%, of the particles of different colors havethe same or similar electrophoretic mobilities. As an example, in asystem having two species of particles, less than 25% of particles ofone species would have the same or similar electrophoretic mobilities asthe particles in the other species.

In an alternative embodiment, one of the colors may be represented by adye dispersed in the phase change material. In this embodiment, theparticles may, for example, all have either a positive charge or allhave a negative charge. When the electric field is applied in adirection that draws the particles towards the surface of theelectrophoretic coating, the section of the electrophoretic coating willappear to a viewer to be the color of the particles. When the electricfield is applied in a direction that draws the particles away from thesurface of the electrophoretic coating, the section of theelectrophoretic coating will appear to a viewer to be the color of thedye. In this embodiment, the phase change material is preferably nottransparent when the phase change material is a solid.

When the electrophoretic coating of the invention is used to form acolored image, the particles of a particular color have a different zetapotential, and hence a different electrophoretic mobility, thanparticles having a different color. For example, each microcapsule cancontain red particles with an average zeta potential of 100 mV, greenparticles with an average zeta potential of 60 mV, and blue particleswith an average zeta potential of 20 mV. To address a section of theelectrophoretic coating to the red state, the phase change material inthe microcapsule is melted and all the particles in the microcapsulesare pulled away from the surface of the electrophoretic coating byapplying an electric field in one direction. Subsequently, the field isreversed for just long enough for the red particles in the microcapsulesto move toward the side of the microcapsule that faces the surface ofthe electrophoretic coating. The green and blue particles will also movein this reversed field, but they will not move as quickly as the redparticles, and thus will be obscured by the red particles.

To address a section of the electrophoretic coating to the green state,all the particles in the microcapsules are pulled away from the surfaceof the electrophoretic coating by applying an electric field in onedirection. Then the field is reversed for just long enough for the redand green particles to move to the side of the microcapsule facing thesurface of the electrophoretic coating. The field is then reversed againand the red particles, moving faster than the green particles, leave thegreen particles exposed at the side of the microcapsule facing thesurface of the electrophoretic display.

Finally, to achieve a blue display, all the particles are pulled towardthe side of the microcapsule that faces the surface of theelectrophoretic coating. The field is then reversed and the blueparticles, lagging behind the red and green particles are exposed on theside of the microcapsule that faces the surface of the electrophoreticcoating.

When forming a colored display, one of the colors optionally may berepresented by a dye dispersed in the phase change material. In thisembodiment, the phase change material is preferably not transparent whenit is a solid.

Once the electrophoretic coating has cooled enough for the phase changematerial to solidify, the image which has been displayed on therewritable medium is maintained as it is even after the application ofthe voltage is stopped and is not erased or disrupted by the presence ofan electric field, electrostatic discharge, or normal electric chargesthat can build up during handling of the rewritable medium. The imagecan be written over and a different image can be generated by heatingdifferent sections of the electrophoretic coating and applying anelectric field to each section such that the direction of the electricfield gradient in the section of the electrophoretic medium has adifferent pattern than that of the first image. To erase the image sothat the rewritable medium has one uniform color, the entire rewritablemedium is heated above the melting point of the phase change materialand a uniform electric field is applied to the entire electrophoreticcoating so that each of the microcapsules in the electrophoretic coatingexperiences an electric field having about the same strength and in theabout the same direction. Alternatively, an image can be erased byheating sections of the electrophoretic coating separately and applyingan electric field to each of the sections individually or line by line,provided that the electric field applied to each section is about thesame strength and is applied in about the same direction for about thesame length of time.

The strength of the electric field needed to address a section of theelectrophoretic coating depends on a number of factors including thetime period in which the electric field is applied to a section of theelectrophoretic coating, the viscosity of the liquefied phase changematerial and the type and charge of particles in the microcapsules. Theperiod of time for which the electric field is applied is typically inthe range of about 0.1 millisecond to about 10 seconds. More typically,the electric field is applied for about 1 millisecond to about 1 second.As the strength of the electric field is increased, the period of timethat the electrophoretic coating needs to be in the electric field tocause the desired migration of the charged particle is decreased.

FIG. 2A shows a schematic illustration of a CD or DVD 16 in anembodiment of an apparatus for forming or erasing an image on arewritable medium. FIG. 2B is a cross-section of the CD or DVD 16 andthe apparatus along line (I). The CD or DVD 16 has a polycarbonatesubstrate 20 with a metal layer 22 on one side of the substrate 20. Theelectrophoretic coating 24 is on top of the metal layer 22 and can beencapsulated in a protective coating 26. The metal layer 22 is connectedto one terminal of a power source (not shown) through a spindle 28 thatis in contact with a drive ring 30. The drive ring 30 is in contact witha metal contact 32 that in turn is in contact with the metal layer 22. Apositive or negative charge can be applied to the metal layer 22 throughthe spindle 28 which is in contact with the power source. The apparatushas an arm 38 that includes a heating element 36 and one or more pointelectrodes 34 that are connected to the other terminal of the powersource. The point electrodes can be touching the surface of theelectrophoretic coating or suspended in close proximity to theelectrophoretic coating. The power source is capable of switching thecharge at each terminal when the electric field is moved to a differentsection of the electrophoretic coat. Thus, data regarding each pixel ofan image can be stored in a data storage device and used to control thepixel color at a particular pixel location by controlling the directionof the electric field applied to that pixel. The spindle 28 can rotatethe disc at a substantially uniform speed so that the electrophoreticcoating is transported into an area that is heated by the heatingelement and heated to a sufficient temperature to liquefy the phasechange material in the microcapsules of the electrophoretic coating andthereafter is transported into the electric field generated by theelectrode. Alternatively, the heating element and electrodes can bepositioned such that a section of the elelctrophoretic coating is heatedat the same time as an electric field is applied. In another alternativeembodiment, the disc remains stationary on the spindle and the arm 38containing the heating element 36 and the electrodes 34 is moved overthe disc at a substantially uniform speed.

FIG. 3 is a schematic representation of an alternative embodiment of anapparatus of the invention that is designed to form or erase an image onan embodiment of a rewritable medium of the invention that does not havea metal layer. In this embodiment, the apparatus includes electrodepairs 42 a and 42 b, 44 a and 44 b, and 46 a and 46 b that are eachconnected to one terminal of a power source and are aligned with eachother above and below the rewritable medium 40. The apparatus alsoincludes a IR heater 50 and a reflector 48 which is aligned with theelectrode pair (42 a, 42 b, 44 a, 44 b, 46 a, and 46 b) such that thetransport rollers 52 a and 52 b move the rewritable medium 40 through anarea that is heated to a sufficient temperature to melt the phase changematerial in the microcapsules 54 by the IR heater 50 and then istransported into the electric field generated by the aligned metalplates of the electrodes (42 a, 42 b, 44 a, 44 b, 46 a, and 46 b).Alternatively, and infrared laser, radio frequency induction, ormicrowave impingement can be used to liquefy the phase change material.

In one embodiment, the apparatus for forming or erasing an imageoperates as follows. The rewritable medium 40 is transported by thetransport unit at the constant speed in the direction indicated by thearrow in FIG. 3. At first, the rewritable medium 40 passes through aspace heated by the heating element 50 sufficiently to melt the phasechange material. The rewritable medium 40 then passes the electrodes 42a, 42 b, 44 a, 44 b, 46 a, and 46 b which apply a directional electricfield to form a predetermined image. Shortly after the applied heat isremoved from the section of the electrophoretic coating, the phasechange material solidifies and a constant image is maintained on therewritable medium 40. A new image can be formed by passing therewritable medium 40 through the apparatus of FIG. 3 again.

In one embodiment, the electrodes (42 a, 42 b, 44 a, 44 b, 46 a, and 46b) may have a size approximately equivalent to the picture element, andthey are capable of scanning in the vertical and lateral directions.

In another embodiment, the electric field may be generated by a line ofelectrodes having approximately the same width as the lateral width ofthe rewritable medium. The line of electrodes may be constructed suchthat the electric field is applied differently for each of pictureelements (image pixels) in the array in the lateral direction of therewritable medium 40.

In another alternative embodiment, the electric field may be generatedby a three parallel lines of electrodes each having approximately thesame width as the lateral width of the rewritable medium. In thisembodiment, the particles in the microcapsules of a first row of pictureelements can migrate in the electric field of the first row ofelectrodes. When the three rows of electrode are move down one line, theparticles in the first row of picture elements then migrates in theelectric field of the second row of electrodes which repeats theelectric field pattern that the first row of electrodes had whenaddressing the first row of picture elements. When the three rows ofelectrode are move again move down one line, the particles in the firstrow of picture elements then migrates in the electric field of the thirdrow of electrodes which, once again, repeats the electric field patternthat the first row of electrodes had when addressing the first row ofpicture elements. Using three rows of electrodes can thus speed up thetime in which the rewritable medium passes through the apparatus to forman image.

In another alternative embodiment, a two dimensional array of electrodescan be used to form an image on the rewritable medium. In thisembodiment, the entire rewritable medium is heated to a temperaturesufficient to liquefy the phase change material. Then the rewritablemedium is placed in the electric fields generated by the electrodes inthe array and the image is formed on the entire rewritable medium in onestep.

In another embodiment of the apparatus of the invention, the rewritablemedium remains stationary and the heating element and electrodes forgenerating the electric field are capable of scanning, preferably inunison, in the vertical and/or lateral directions such the heatingelement heats a section of the electrophoretic coating. The electrodesused to produce the electric field are moved into position above andbelow a section of the rewritable medium either simultaneously with theheating element or just after the section has been heated so that theyproduce an electric field in that section of the rewritable mediumbefore the phase change material can solidify.

EXAMPLES

For the purposes of illustration only,

I. Formation of Non-Encapsulated Phase Change Particle DispersionMicrocapsules

A dye or combination of dyes that are soluble in the liquid phase of thephase change material is chosen. The phase change material can be anyone of the following materials: 1-doosanol, paraffin wax,1-hexacconsanol, n-tetratetracontane, 1-tricontanol, or n-pencontane.

Titania particles with diameters between 10 nm to about 5 microns areused for dispersion in the microcapsules. The particles should besmaller than the microcapsules that are to be formed and occupy betweenabout 0.1% and about 20% of the volume of the microcapsule.

The phase change material is heated until it liquefies and has aviscosity suitable for dispersing the particles utilizing conventionaldispersion techniques, such as high speed/shear mixing or ultrasonictreatment.

An IR absorbing material is added to the liquid phase change material.The IR absorbing material can be a pure compound or mixture of compoundsprovided that it is soluble in the liquid phase change material. The IRabsorbing material is selected such that it absorbs the IR laserradiation, converts the IR radiation into heat sufficient to melt thephase change material.

The appropriate amount of particles are added to the liquid phase changematerial with continued stirring at a rate sufficient to maintain auniform dispersion.

The uniform liquid dispersion is rapidly cooled to entrap the particleswithin solid phase change material microcapsules by one of the followingmethods:

a) Spraying the hot liquid dispersion into a cold atmosphere or coldliquid, in which the phase change material is insoluble;

b) Pouring the hot liquid dispersion into a rapidly stirring coldliquid, in which the phase change material is insoluble, such that fineparticles (i.e., microcapsules) are formed, collecting the solidifiedmicrocapsules, and reducing the microcapsule size using conventionalgrinding or milling techniques; or

c) Extruding the hot liquid dispersion through an orifice such that adroplet forms, then releases and drops through a cold atmosphere thatcauses solidification into a microcapsule. Microcapsules can then becollected in a container or liquid medium.

After solidification of the phase change material to form microcapsules,additional grinding and milling of the microcapsules can be carried outif required to achieve the appropriate size.

II. Formation of Encapsulated Microcapsules Via Co-Extrusion

A hot liquid dispersion of particles in a phase change material isextruded through a central orifice such that a droplet forms and thenanother polymeric material, which has a higher melting than Phase ChangeMaterial, is extruded around the phase change material particledispersion droplet.

The co-extruded droplet is then release and drop through a coldatmosphere causing solidification of a polymeric shell around the phasechange material particle dispersion droplet.

III. Formation of Encapsulated Microcapsules Via Vapor Phase Coating

A polymeric material, such as paralene, is heated such that itdecomposes into vapor of reactive gaseous fragments. The solid phasechange material microcapsules prepared as in Example I are exposed tothis vapor. The solid phase change material microcapsules may be tumbledin a chamber or fluidized bed techniques may be used to ensure a uniformexposure over the entire microcapsule surface. The reactive gaseousfragments will absorb on the microcapsule surface and recombine into apolymeric material that coats the microcapsule.

IV. Formation of Encapsulated Microcapsules Via Complex Coacervation

Acacia is dissolved in water with stirring at room temperature. Anyinsoluble material is removed by centrifuge techniques.

The purified acacia solution is transferred into a reactor vessel andheated to 40° C. The solution is then agitate using a paddle stirrer.Gelatin is added to the stirring solution such that no clumping of thesolid occurs. The resulting solution is stirred for 30 minutes.

A liquid phase change material particle dispersion is added to thestirring solution, and the solution is stirred at a rate sufficient toform a uniform particle dispersion but below the rate that causesfoaming. The pH is reduced to approximately 4 over several minutes usingan aqueous 10% acetic acid solution. Water that has been preheated to40° C. is added to the solution, and the temperature is reduced to 10°C. A 37% aqueous formalin solution is added, and the solution is stirredfor an additional 60 minutes. Sodium carboxymethyl-cellulose is added tothe solution, and the pH is increased to 10.0 by the addition of aqueous20 wt. % sodium hydroxide. The temperature is increased to 40° C. andstirring is continued for about an hour. The resulting slurry is allowedto slowly cool overnight to ambient temperature with stirring. Thecoated microcapsules are collected using conventional filtrationtechniques.

V. Formation of Encapsulated Microcapsules Via InterfacialPolymerization

This method can be utilized only when the phase change material isparaffin wax or hydrocarbons. Phase change materials that have alcoholicgroups cannot be used because they react with the acid chloride monomer.

The phase change material is heated until it liquefies with a viscositysuitable for dispersing the electrophoretic particles utilizingconventional dispersion techniques, such as high speed/shear mixing orultrasonic treatment. Under an inert atmosphere, an alkyl diacidchloride, such as sebacoyl chloride, is added to the liquefied phasechange material along with electrophoretic particles. The inertatmosphere is used to minimize potential reaction of the diacid chloridewith moisture at elevated temperatures.

The liquid dispersion is rapidly cooled to entrap the electrophoreticparticles within solid phase change material microcapsules by one of thefollowing methods:

a) Spraying the hot liquid dispersion into a cold inert atmosphere or acold liquid, in which the phase change material is insoluble and isnon-reactive to the diacid chloride;

b) Pouring the hot liquid dispersion into a cold liquid, in which thephase change material is insoluble and which is non-reactive to thediacid chloride, such that fine particles (i.e., microcapsules) areformed, collecting the solid microcapsules, and reducing themicrocapsule size using conventional grinding or milling techniques; or

c) Extruding the hot liquid dispersion through an orifice such that adroplet forms, then releases, and drops through a cold inert atmospherethat causes solidification into a microcapsule. Microcapsules are thencollected in a container or liquid medium.

Phase change particle dispersion microcapsules are added into a stirringaqueous solution containing an alkyldiamine, such as 1,6-diaminohexane.While stirring, the dispersion is heated sufficiently to melt the phasechange particle dispersion microcapsules. The stirring rate is adjustedto form a uniform dispersion of suitably sized droplets. The solution isstirred and heating for about an hour. A polymer shell will form at thewater/phase change material interface and eventually encapsulate theliquid phase change particle dispersion microcapsules. The slurry iscooled with stirring to ambient temperature, and the coatedmicrocapsules are collected using conventional filtration techniques.

VI. Formation of Encapsulated Microcapsules Via In-Situ Polymerization

A dilute aqueous mixture of ethylene-co-maleic anhydride, resorcinol,and urea is formed. The mixture is stirred and the pH is quickly adjustto 3.5 using 25 wt. % sodium hydroxide. Phase change particle dispersionmicrocapsules are added. An aqueous 37 wt. % formaldehyde solution isadded to the rapidly stirring dispersion, and the temperature is thenincreased to about 55° C. The temperature and stirring are maintainedfor several hours. The slurry is then cooled with stirring to ambienttemperatures, and the encapsulated microcapsules are collected usingconventional filtration techniques.

VII. Formation of Non-Encapsulated Microcapsules in a Matrix This MethodUtilizes a Binder Matrix as an Encapsulation Medium.

The phase change particle dispersion microcapsules are dispersed into abinder matrix solution utilizing conventional techniques. The matrixsolution (and solvent if present) is selected so that the phase changematerial has minimal (preferred no) solubility in it. The processingparameters are adjusted to achieve a stable and uniformmatrix/microcapsule dispersion.

A substrate is coated with the above dispersion, and the dispersion iscured by standard techniques (e.g., solvent evaporation, UV-cure,condensation or free-radical polymerization, etc.). The liquefied phasechange material must have minimal (preferably no) solubility in thecured matrix.

OTHER EMBODIMENTS

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of the specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope of theinvention being indicated by the following claims.

1. An apparatus for forming or erasing an image on the rewritable mediumfor visual display comprising an electrophoretic coating on a substrate,wherein the electrophoretic coating comprises microcapsules comprisingcharged particles of one or more colors suspended in a phase changematerial that has a melting temperature in the range of between about30° C. and about 200° C., wherein particles of at least one color have apositive or a negative charge, and a polymer matrix, wherein themicrocapsules are distributed in the polymer matrix comprising: a) aheating element; b) at least one electrode; c) a means for positioningthe heating element to heat a section of the electrophoretic coating;and d) a means for positioning the at least one electrode above thesurface of the section of the electrophoretic coating.
 2. The apparatusof claim 1, wherein the at least one electrode is positioned above thesurface of a section of the electrophoretic medium that issimultaneously heated by the heating element.
 3. The apparatus of claim1, wherein the heating element is positioned to heat a section of theelectrophoretic medium before the at least one electrode is positionedabove the surface of said section of the electrophoretic coating.
 4. Theapparatus of claim 1, wherein the heating element is an infrared laser,an infrared lamp, an RF induction heater, or a microwave heating device.5. The apparatus of claim 1, wherein the rewritable medium includes ametal layer between the electrophoretic coating and the substrate; andthe apparatus further comprises a power source connected to theelectrode and the metal layer.
 6. The apparatus of claim 1, wherein theapparatus comprises at least two electrodes each comprising a metalplate connected to a power supply, wherein one metal plate is placedabove the surface of the electrophoretic coating and the other electrodeis placed on the opposite side of the electrophoretic coating.
 7. Theapparatus of claim 1, comprising a line of electrode pairs, wherein oneelectrode of an electrode pair is placed below the rewritable medium andthe other electrode of the electrode pair is placed above the rewritablemedium.
 8. The apparatus of claim 1, comprising three parallel lines ofelectrode pairs, wherein one electrode of an electrode pair is placedbelow the rewritable medium and the other electrode of the electrodepair is placed above the rewritable medium.
 9. The apparatus of claim 7or 8, wherein the means for positioning the heater and the electrode isa device that moves the heater and the electrodes horizontally orvertically over the rewritable medium.
 10. The apparatus of claim 1,wherein the means for positioning the heater and the means forpositioning the electrode is a means for moving the rewritable medium.11. The apparatus of claim 10, wherein the substrate is a polycarbonatedisc and the means for moving the rewritable medium is a drive ringwhich moves the disc.
 12. The apparatus of claim 10, wherein thesubstrate is paper and the means for moving the rewritable medium is atransport roller.